Removed heroku gem from template Gemfile.

It hasn't been used since commit bc9abe removed support for it anyway.
Remove shotgun dependency

Fixes gma/nesta#125
Only install shotgun in development.
Dev depends on mr-sparkle.
Updated all gems/dependencies.
Don't swallow LoadError when loading app.rb.

If we try and load app.rb locally, or from a theme, it's quite possible
that the code inside app.rb will raise LoadError, which should be raised
in order to hide the problem from the user.
Removed Travis build status from README.

The build usually fails due to the environment on Travis not coping for
some reason, so pointing out that Nesta's tests aren't passing isn't
accurate or useful.

It's currently failing on 1.9.3 because rubygems needs updating on
Travis. Ruby is such a frigging ghetto; rubygems should be far more
mature than this by now. But such is our lot...
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Removed Travis build status from README.
  Don't swallow LoadError when loading app.rb.
  Updated all gems/dependencies.
  Dev depends on mr-sparkle.
  Only install shotgun in development.
  Remove shotgun dependency
  Removed heroku gem from template Gemfile.

Conflicts:
	nesta.gemspec
	templates/Gemfile