travis: Separate requirement files

This way we can update pip independently before installing the rest, and
avoid installing codecov (and thus coverage which attempts to build C
extension) where it's not needed.

scripts/dev/ci/requirements-codecov.txt

@@ -0,0 +1,3 @@
+codecov==2.0.3
+coverage==4.0.3
+requests==2.10.0

scripts/dev/ci/requirements-pip.txt

@@ -1,3 +1 @@
 pip==8.1.1  # rq.filter: != 8.1.2
-codecov==2.0.3
-tox==2.3.1

scripts/dev/ci/requirements-tox.txt

@@ -0,0 +1,4 @@
+pluggy==0.3.1
+py==1.4.31
+tox==2.3.1
+virtualenv==15.0.1

scripts/dev/ci/travis_install.sh

@@ -54,7 +54,7 @@ brew_install() {
 
 pip_install() {
     # this uses python2
-    travis_retry sudo -H python -m pip install -r scripts/dev/ci/requirements-travis.txt
+    travis_retry sudo -H python -m pip install -r scripts/dev/ci/requirements-$1.txt
 }
 
 npm_install() {
@@ -84,17 +84,20 @@ elif [[ $TRAVIS_OS_NAME == osx ]]; then
     # Disable App Nap
     defaults write NSGlobalDomain NSAppSleepDisabled -bool YES
     brew_install python3 pyqt5
-    pip_install
+    pip_install pip
+    pip_install tox
     check_pyqt
     exit 0
 fi
 
 pyqt_pkgs="python3-pyqt5 python3-pyqt5.qtwebkit"
 
-pip_install
+pip_install pip
+pip_install tox
 
 case $TESTENV in
     py34-cov)
+        pip_install codecov
         apt_install xvfb $pyqt_pkgs libpython3.4-dev
         check_pyqt
         ;;