Set defaults for INSTAL_DEST LSB_RELEASE and PACKAGES

in case the overrides in `env.global` don't set them

Author: meatballhat

.travis.yml

@@ -15,16 +15,18 @@ env:
   global:
     - VERSION='3.6-dev'
     - ALIAS='nightly'
-    - INSTALL_DEST=/opt/python
-    - PACKAGES='pip numpy nose pytest mock wheel'
-    - LSB_RELEASE="$(lsb_release -rs)"
 
 install:
   - pushd /opt/pyenv/
   - sudo git checkout master
   - sudo git pull
   - popd
 
+before_script:
+  - ": ${INSTALL_DEST:=/opt/python}"
+  - ": ${LSB_RELEASE:=$(lsb_release -rs)}"
+  - ": ${PACKAGES:=pip numpy nose pytest mock wheel}"
+
 script: ./build-python
 
 after_success: ./create-archive