Enhancement: Speed up builds as much as possible

[localheinz]

This PR

• indents .travis.yml consistently (unrelated)
• provides feedback quickly if it has failed, using the fast_finish option
• moves update of Composer itself to before_install section (unrelated)
• runs composer install instead of composer update (unrelated)
• checks links on PHP 5.6 only
• builds docs on PHP 5.6 only

Before

See https://travis-ci.org/justinrainbow/json-schema/builds/70676013.

After

https://travis-ci.org/justinrainbow/json-schema/builds/70680917.

[localheinz]

Seems like most of the time is still spent on installing dependencies with Composer. If links wouldn't needed to be checked on Travis, container-based infrastructure could be enabled, cutting the whole build down to probably around 2 minutes.

[bighappyface]

@localheinz would you push the link check and docs build below phpunit for the fastest fail on PHP 5.6? We probably should have done that a long time ago considering it's pointless to build the docs for broken code.

Overall I think this is a great improvement.

[localheinz]

@bighappyface

👍

[bighappyface]

+1