try to use travis's builtin deploy workflow

Author: nikomatsakis

.travis.yml

@@ -15,3 +15,10 @@ notifications:
 env:
   global:
     secure: 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
+deploy:
+  provider: pages
+  skip-cleanup: true
+  github-token: $GITHUB_TOKEN
+  local-dir: book
+  on:
+    branch: master

ci/github_pages.sh

@@ -6,7 +6,6 @@ BOOK_DIR=book
 # Only upload the built book to github pages if it's a commit to master
 if [ "$TRAVIS_BRANCH" = master -a "$TRAVIS_PULL_REQUEST" = false ]; then
     mdbook build 
-    ghp-import -p $BOOK_DIR
 else
     echo Skipping 'mdbook build' because this is not master or this is just a PR.
 fi