Add back the spirit sublibrary of boost.

We apparently need it from the serialization sublibrary.
Add forgotten header file.
Merge pull request #116 from bangerth/add-missing-spirit-lib

Add back the spirit sublibrary of boost.
Only test indentation on pull requests.
Merge pull request #117 from luca-heltai/travis_ci_indent_check

Only test indentation on pull requests. This closes issue #89, where we discussed about possibly testing with Travis CI the indentation of the pull request.

Since most of the pull requests are failing anyway on Travis (time limit for a pull request is 25 minutes, and we hit it pretty regularly), this pull request configures Travis CI to only check indentation in pull requests, and do the full build only when the pull request is merged on master.
Removed all builds. Only indent test.
Moved download and install of astyle into before_install section. Renamed files.
Merge pull request #118 from luca-heltai/fix_travis_ci

Removed all builds. Only indent test.

I implemented here Matthias' suggestion. Removing all unnecessary comments and lines from the Travis CI configuration scripts. This should also make it much faster, since we don't need to install anything anymore (except astyle). This is the one that closes #89.