Fix Travis

There were two causes of breakage:

 - using pip 6.0.7, which doesn't properly install things in the right order
 - requirements-install.sh not properly handling the case where SIX_VERSION
   isn't defined

Author: gsnedders

.travis.yml

@@ -19,7 +19,7 @@ env:
   - SIX_VERSION=1.9 USE_OPTIONAL=true
 
 install:
-  - bash requirements-install.sh
+  - ./requirements-install.sh
 
 script:
   - if [[ $TRAVIS_PYTHON_VERSION == pypy* ]]; then py.test; fi

requirements-install.sh

@@ -1,17 +1,14 @@
-#!/bin/bash -e
+#!/bin/bash -ex
 
-if [[ $USE_OPTIONAL != "true" && $USE_OPTIONAL != "false" ]]; then
-  echo "fatal: \$USE_OPTIONAL not set to true or false. Exiting."
-  exit 1
-fi
+pip install pip==6.1.0
 
 pip install -U -r requirements-test.txt
 
 if [[ $USE_OPTIONAL == "true" ]]; then
   pip install -U -r requirements-optional.txt
 fi
 
-if [[ $SIX_VERSION != "false" ]]; then
+if [[ $SIX_VERSION ]]; then
   pip install six==$SIX_VERSION
 fi