Assign RAILS_ENV only if needed. Should fix #17

Fixes annoying message which appears on rails `>= 4.1`
```
.../gems/jumpup-0.0.4/lib/tasks/integrate.rake:76: warning: already initialized constant RAILS_ENV
.../gems/jumpup-0.0.4/lib/tasks/integrate.rake:76: warning: previous definition of RAILS_ENV was here
```