Fix command push

[LucianoPC]

• push was using 'exec' to call bash commands
  but this function call the bash command into
  another process and never back to the ruby
  process.
• I change to use the function 'system' for
  each bash function, because this function
  call the bash command into another process
  and before returns to the ruby process

[coveralls]

Coverage decreased (-2.5%) to 88.608% when pulling f93de8b on LucianoPC:fix_command_push into be17fe9 on lemurheavy:master.

[coveralls]

Coverage decreased (-1.9%) to 89.254% when pulling f93de8b on LucianoPC:fix_command_push into be17fe9 on lemurheavy:master.

[nickmerwin]

Thanks @LucianoPC!