Use --pre flag for Travis pip installs.

This way we can help NumPy and others test their pre-releases.

.travis.yml

@@ -35,12 +35,16 @@ notifications:
     on_failure: always  # options: [always|never|change] default: always
     on_start: false     # default: false
 
-python:
-  - 2.6
-  - 2.7
-  - 3.3
-  - 3.4
-  - 3.5
+matrix:
+  include:
+    - python: 2.6
+    - python: 2.7
+      env: PIP_FLAGS="--pre"
+    - python: 2.7
+      env: MINIMUM_REQUIREMENTS=1
+    - python: 3.4
+    - python: 3.5
+      env: PIP_FLAGS="--pre"
 
 before_install:
     - ccache -s

tools/travis_before_install.sh

@@ -55,9 +55,9 @@ pip install --retries 3 -q wheel flake8 coveralls nose
 for requirement in $WHEELBINARIES; do
     WHEELS="$WHEELS $(grep $requirement requirements.txt)"
 done
-pip install --retries 3 -q $WHEELHOUSE $WHEELS
+pip install --retries 3 -q $PIP_FLAGS $WHEELHOUSE $WHEELS
 
-pip install --retries 3 -q -r requirements.txt
+pip install --retries 3 -q $PIP_FLAGS -r requirements.txt
 
 # Show what's installed
 pip list