Add CodeSniffer as composer development dependency.
Add composer.lock file.
Install composer dependencies on Travis CI.
Add ruleset for phpseclib coding standard.
Run code sniffer on PHP 5.5 environment on Travis CI.
Only install composer dependencies in PHP 5.5 environment.

There is currently no point in installing in any other environment.
Merge pull request #186 from bantu/code-sniffer

Introducing Code Sniffer / Ensuring coding standard

* bantu/code-sniffer:
  Only install composer dependencies in PHP 5.5 environment.
  Run code sniffer on PHP 5.5 environment on Travis CI.
  Add ruleset for phpseclib coding standard.
  Install composer dependencies on Travis CI.
  Add composer.lock file.
  Add CodeSniffer as composer development dependency.