gem version updates -- locomotive_cms 2.5.5, rails 3.2.19, compass 0.12.7, compass-rails 1.1.7

Update documentation so that new users can copy/paste tutorial and have a functioning application.

Explicitly define 'compass' gem version per issue ["Setting up Locomotive Engine Gone horribly wrong"](https://stackoverflow.com/questions/25416557/setting-up-locomotive-engine-gone-horribly-wrong):
```
'compass',  '~> 0.12.7'
```

Use most recent and secure versions of gems:
```
'rails', '3.2.19'
'locomotive_cms', '~> 2.5.5'
'compass-rails',  '~> 1.1.7'
```