Install Rake and Bundler in isolation when missing

Some Rubies need Rake or Bundler for the installation process. However,
since the host Ruby version might not have those gems installed, install
them in a temporary GEM_HOME and add their executables to PATH.

Fixes #426

bin/ruby-build

@@ -703,10 +703,29 @@ rake() {
   if [ -e "./Gemfile" ]; then
     bundle exec rake "$@"
   else
+    isolated_gem_dependency "rake --version" rake -v '~> 10.1.0'
     command rake "$@"
   fi
 }
 
+bundle() {
+  isolated_gem_dependency "bundle --version" bundler -v '~> 1.3.5'
+  command bundle "$@"
+}
+
+isolated_gem_dependency() {
+  if ! command $1 &>/dev/null; then
+    shift 1
+    isolated_gem_install "$@"
+  fi
+}
+
+isolated_gem_install() {
+  export GEM_HOME="${PWD}/.gem"
+  export PATH="${GEM_HOME}/bin:${PATH}"
+  gem install "$@"
+}
+
 version() {
   echo "ruby-build ${RUBY_BUILD_VERSION}"
 }

test/build.bats

@@ -142,7 +142,8 @@ OUT
 install_package "mruby-1.0" "http://ruby-lang.org/pub/mruby-1.0.tar.gz" mruby
 DEF
 
-  stub rake true
+  stub gem false
+  stub rake '--version : true' true
 
   run ruby-build "definition" "$INSTALL_ROOT"
   assert_success
@@ -162,8 +163,12 @@ DEF
 install_package "rubinius-2.0.0" "http://releases.rubini.us/rubinius-2.0.0.tar.gz" rbx
 DEF
 
+  stub gem false
   stub rake false
-  stub bundle ' : echo bundle >> build.log' \
+  stub bundle \
+    '--version : true' \
+    ' : echo bundle >> build.log' \
+    '--version : true' \
     " exec rake install : { cat build.log; echo bundle \"\$@\"; } >> '$INSTALL_ROOT/build.log'"
 
   run ruby-build "definition" "$INSTALL_ROOT"

test/test_helper.bash

@@ -3,8 +3,10 @@ export TMP="$BATS_TEST_DIRNAME/tmp"
 if [ "$FIXTURE_ROOT" != "$BATS_TEST_DIRNAME/fixtures" ]; then
   export FIXTURE_ROOT="$BATS_TEST_DIRNAME/fixtures"
   export INSTALL_ROOT="$TMP/install"
-  export PATH="$BATS_TEST_DIRNAME/../bin:$PATH"
-  export PATH="$TMP/bin:$PATH"
+  PATH=/usr/bin:/usr/sbin:/bin/:/sbin
+  PATH="$BATS_TEST_DIRNAME/../bin:$PATH"
+  PATH="$TMP/bin:$PATH"
+  export PATH
 fi
 
 teardown() {