Fetch the pull request number using @sassbot's credentials

This avoids running into GitHub's low rate limits for unauthenticated
requests.

.travis.yml

@@ -1,15 +1,16 @@
 sudo: false
 language: ruby
 cache: bundler
+
 install:
   # If we're running for a pull request, check out the revision of sass-spec
   # referenced by that pull request.
   - |
     if [ ! -z "$TRAVIS_PULL_REQUEST" -a "$TRAVIS_PULL_REQUEST" != false ]; then
+      ref=$(extra/sass-spec-ref.sh)
       mkdir sass-spec
       git -C sass-spec init
-      git -C sass-spec pull --depth=1 git://github.com/sass/sass-spec \
-        $(extra/sass-spec-ref.sh)
+      git -C sass-spec pull --depth=1 git://github.com/sass/sass-spec "$ref"
       bundle config local.sass-spec "$(pwd)/sass-spec"
     fi
 

extra/sass-spec-ref.sh

@@ -18,7 +18,15 @@ fi
 
 >&2 echo "Fetching pull request $TRAVIS_PULL_REQUEST..."
 
-JSON=$(curl -L -sS https://api.github.com/repos/sass/sass/pulls/$TRAVIS_PULL_REQUEST)
+url=https://api.github.com/repos/sass/sass/pulls/$TRAVIS_PULL_REQUEST
+if [ -z "$GITHUB_AUTH" ]; then
+    >&2 echo "Fetching pull request info without authentication"
+    JSON=$(curl -L -sS $url)
+else
+    >&2 echo "Fetching pull request info as sassbot"
+    JSON=$(curl -u "sassbot:$GITHUB_AUTH" -L -sS $url)
+fi
+>&2 echo "$JSON"
 
 RE_SPEC_PR="sass\/sass-spec(#|\/pull\/)([0-9]+)"