Add script/bootstrap

This just runs Bundler with the right options. Maybe it will do more in
the future, like update submodules.

README.md

@@ -141,7 +141,7 @@ But for development you are going to want to checkout out the source. To get it,
 
     git clone https://github.com/github/linguist.git
     cd linguist/
-    bundle install
+    script/bootstrap
 
 To run the tests:
 

script/bootstrap

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "$0")/.."
+
+bundle check --path vendor/gems > /dev/null 2>&1 || \
+    bundle install --local --path vendor/gems

script/cibuild

@@ -1,4 +1,7 @@
 #!/bin/sh
+
+set -e
+
 if [ -d /usr/share/rbenv/shims ]; then
   export PATH=/usr/share/rbenv/shims:$PATH
   export RBENV_VERSION=2.1.2-github
@@ -11,11 +14,9 @@ ruby -v
 bundle -v
 set +x
 
-mkdir -p ./vendor/gems
-
 # Clean out any unversioned files
 git clean -fd
 
-bundle install --local --path ./vendor/gems
+script/bootstrap
 bundle exec rake samples
 bundle exec rake