Adding `travis_wait` for `mvn install`

Adding `travis_wait` to `.travis.yml` to write a log every minute while executing `mvn install` (with default timeout of 20 min.). This _should_ help to get rid all intermittent build failures due to timeouts installing dependencies.

Examples of failed builds due to timeouts:
* https://travis-ci.org/fakemongo/fongo/builds/29705211
* https://travis-ci.org/fakemongo/fongo/builds/29705060
* https://travis-ci.org/fakemongo/fongo/builds/29694848

If this doesn't help, further steps could be increasing the timeout and/or adding `travis_retry`. Please see http://docs.travis-ci.com/user/build-timeouts/

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@twillouer please review, merge and delete branch.