Simplify Ruby version management for deployment

[abitrolly]

Currently, Ruby version is specified in three places:

1. https://github.com/minsk-hackerspace/hackerspace.by/blob/master/.ruby-version

2. https://github.com/minsk-hackerspace/hackerspace.by/blob/9c52c3eebfe4d171eb8424d79b4ccb8d05b53e80/.travis.yml#L6

3. config/deploy.rb - https://github.com/minsk-hackerspace/hackerspace.by/commit/7456df682f6a1199bc85a25d285c51d763be0102

• [ ] Need to check what happens if the version is removed from .travis.yml.
• [ ] Then it may be possible to move it to Gemfile to remove extra file https://travis-ci.community/t/support-ruby-version-in-gemfile/6898 This can be done by invoking rvm second time explicitly. It still needs to be explicitly called in deploy script (#430)
• [ ] Use the same method for Vagrant (provision Ruby)