Formalize PR code review policy w/ pullapprove.com

 - Right now:
   - At least one person form SI (@zbeekman, @rouson, @afanfa) must approve a PR
   - Approver may be the PR Author
   - This is the regexp to match approval via comments:
     `'^([Aa]pproved|:\+1:|:shipit:|:ship: it|LGTM|:zap:|:boom:|:clap:|:bow:)'`
   - This is the regexp to reject via comment:
     `'^([Nn]ope|[Nn]ope\.jpg|[Dd]enied|[Rr]ejected|:\-1:)'`
   - Approve or reject at pullapprove.com as well

Author: zbeekman

.gitattributes

@@ -24,6 +24,7 @@ install_prerequisites/build text
 
 # Prevent dev-ops files from making it into the release archives
 .travis.yml export-ignore
+.pullapprove.yml export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
 developer-scripts export-ignore

.pullapprove.yml

@@ -0,0 +1,11 @@
+approve_by_comment: true
+approve_regex: '^([Aa]pproved|:\+1:|:shipit:|:ship: it|LGTM|:zap:|:boom:|:clap:|:bow:)'
+reject_regex: '^([Nn]ope|[Nn]ope\.jpg|[Dd]enied|[Rr]ejected|:\-1:)'
+reset_on_push: true
+author_approval: default
+reviewers:
+    required: 1
+    members:
+        - afanfa
+        - rouson
+        - zbeekman