Show first seven char of commit hash (#566)

* Show first seven char of commit hash

This is to follow the same convention of using commit hash so that in
one glance we can detect if we are the desired commit or not.

* Update tests

app/helpers/pages_helper.rb

@@ -7,6 +7,6 @@ def git_commit_sha
 
   def git_commit_sha_short
     full_sha = git_commit_sha
-    full_sha[-7..]
+    full_sha[...7]
   end
 end

spec/system/pages_spec.rb

@@ -19,7 +19,7 @@
 
   context "when .source_version file does not exist" do
     let(:sha) { "94d92356828a56db25fccff9d50f41c525eead5x" }
-    let(:expected_text) { "5eead5x" }
+    let(:expected_text) { "94d9235" }
 
     before do
       # stub this method since we need to control what the sha actually is
@@ -31,7 +31,7 @@
 
   context "when .source_version file exists" do
     let(:sha) { "94d92356828a56db25fccff9d50f41c525eead5y" }
-    let(:expected_text) { "5eead5y" }
+    let(:expected_text) { "94d9235" }
 
     before { `cd #{Rails.root} && echo #{sha} > .source_version` }