.travis.yml: Install using pip once

The default `install:` phase of Travis CI does
`pip install -r requirements.txt`, which was
duplicated by the custom `before_script:` block.

Place `coala-bears` at the top of requirements.txt
to ensure that its pins are respected.

Closes coala#496
Related to coala/meta#123

.travis.yml

@@ -14,13 +14,15 @@ addons:
       - libssl-dev
       - python3-dev
 
-before_script:
-  - pip install coala-bears
-  - pip install -r requirements.txt
-  - pip install -r test-requirements.txt
+before_install:
+  - printf '%s\n%s\n%s\n%s\n'
+           "git+https://github.com/coala/coala"
+           "git+https://github.com/coala/coala-bears"
+           "$(cat test-requirements.txt)"
+           "$(cat requirements.txt)"
+           > requirements.txt
 
 script:
-  - sed -i.bak '/bears = GitCommitBear/d' .coafile
   - coala --non-interactive -V
   - python -m pytest
   - docker build -t "corobo" .