move build dependencies to dedicated files

.travis.yml

@@ -42,10 +42,8 @@ before_install:
   - chmod +x ./cc-test-reporter
 
 install:
-  - if [[ $TRAVIS_PYTHON_VERSION == '2.6' ]]; then travis_retry pip install unittest2 'isort<4.3' 'inflect<0.3.1' 'coveralls<1.3.0' 'pyopenssl<18'; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == '3.2' ]]; then travis_retry pip install 'requests<2.11' 'coverage<4' 'coveralls<1'; else travis_retry pip install 'coverage' 'coveralls'; fi
+  - if [[ -e build-requirements-${TRAVIS_PYTHON_VERSION}.txt ]]; then travis_retry pip install -r build-requirements-${TRAVIS_PYTHON_VERSION}.txt; else travis_retry pip install -r build-requirements.txt; fi
   - travis_retry pip install -r requirements.txt
-  - travis_retry pip install -r build-requirements.txt
   - |
       if [[ $TRAVIS_PYTHON_VERSION == '2.6' || $TRAVIS_PYTHON_VERSION == '2.7' ]]; then
           travis_retry pip install --global-option=build_ext --global-option="-I/usr/include/x86_64-linux-gnu" m2crypto;

build-requirements-2.6.txt

@@ -0,0 +1,8 @@
+isort<4.3
+inflect<0.3.1
+pyopenssl<18
+unittest2
+coverage
+coveralls<1.3.0
+pylint
+diff_cover

build-requirements-3.2.txt

@@ -0,0 +1,5 @@
+requests<2.11
+coverage<4
+coveralls<1
+pylint
+diff_cover

build-requirements.txt

@@ -1,2 +1,4 @@
+coverage
+coveralls
 pylint
 diff_cover