My.cnf Optimization For Mysql Errors

I am getting some mysql error nowadays.My vps at digital ocean 5$ plan.1 cpu , 512 mb ram , centos 6.5 , apache , mysql , nginx, cpanel.Just 2 wordpress sites in it.

Here is my.cnf

[mysqld]
     port            = 3306
     socket          = /var/lib/mysql/mysql.sock
     default-storage-engine= MyISAM
     max_connections = 150 
     key_buffer = 16K
     max_allowed_packet = 1M
     table_open_cache = 2000
     sort_buffer_size = 64K
     read_buffer_size = 256K
     read_rnd_buffer_size = 256K
     net_buffer_length = 2K
     thread_stack = 64K
     log-slow-queries= /var/lib/mysql/slow.log
     log-error = /var/lib/mysql/error.log
     thread_cache_size = 4 
     query_cache_type = 1
     query_cache_size = 16M
     query_cache_limit = 4M



     # For low memory, InnoDB should not be used so keep skip-innodb uncommented unless required
     skip-innodb

     # Uncomment the following if you are using InnoDB tables
     #innodb_data_home_dir = /var/lib/mysql/
     #innodb_data_file_path = ibdata1:10M:autoextend
     #innodb_log_group_home_dir = /var/lib/mysql/
     #innodb_log_arch_dir = /var/lib/mysql/
     # You can set .._buffer_pool_size up to 50 - 80 %
     # of RAM but beware of setting memory usage too high
     #innodb_buffer_pool_size = 16M
     #innodb_additional_mem_pool_size = 2M
     # Set .._log_file_size to 25 % of buffer pool size
     #innodb_log_file_size = 5M
     #innodb_log_buffer_size = 8M
     #innodb_flush_log_at_trx_commit = 1
     #innodb_lock_wait_timeout = 50

     [mysqldump]
     quick
     max_allowed_packet = 16M

     [mysql]
     no-auto-rehash
     # Remove the next comment character if you are not familiar with SQL
     #safe-updates

     [isamchk]
     key_buffer = 8M
     sort_buffer_size = 8M

     [myisamchk]
     key_buffer = 8M
     sort_buffer_size = 8M

     [mysqlhotcopy]
     interactive-timeout

Here is mysqltuner output : http://i.imgur.com/DpDs2UK.png

Last mysql error log :

140818 14:01:15 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
140818 14:01:15 [Warning] Using unique option prefix key_buffer instead of key_buffer_size is deprecated and will be removed in a future release. Please use the full name instead.
140818 14:01:15 [Warning] option 'thread_stack': unsigned value 65536 adjusted to 131072
140818 14:01:15 [Warning] The syntax '--log-slow-queries' is deprecated and will be removed in a future release. Please use '--slow-query-log'/'--slow-query-log-file' instead.
140818 14:01:15 [Note] Plugin 'FEDERATED' is disabled.
140818 14:01:15 [Note] Plugin 'InnoDB' is disabled.
140818 14:01:15 [Note] Server hostname (bind-address): '0.0.0.0'; port: 3306
140818 14:01:15 [Note]   - '0.0.0.0' resolves to '0.0.0.0';
140818 14:01:15 [Note] Server socket created on IP: '0.0.0.0'.
140818 14:01:15 [Note] Event Scheduler: Loaded 0 events
140818 14:01:15 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.37-cll'  socket: '/var/lib/mysql/mysql.sock'  port: 3306  MySQL Community Server (GPL)

Can you help me to fix , my.cnf file.Mysql constantly shutting down itself.I was using Innodb.Here is my sql log for shutdown error :

140818 13:13:31 InnoDB: Initializing buffer pool, size = 16.0M
InnoDB: mmap(17170432 bytes) failed; errno 12
140818 13:13:31 InnoDB: Completed initialization of buffer pool
140818 13:13:31 InnoDB: Fatal error: cannot allocate memory for the buffer pool
140818 13:13:31 [ERROR] Plugin 'InnoDB' init function returned error.
140818 13:13:31 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
140818 13:13:31 [ERROR] Unknown/unsupported storage engine: InnoDB
140818 13:13:31 [ERROR] Aborting

140818 13:13:31 [Note] /usr/sbin/mysqld: Shutdown complete