Do not build the doc on nightly

.travis.yml

@@ -2,16 +2,3 @@ language: rust
 
 rust:
   - nightly
-
-after_success: |
-  [ $TRAVIS_BRANCH = master ] &&
-  [ $TRAVIS_PULL_REQUEST = false ] &&
-  cargo doc &&
-  echo '<meta http-equiv=refresh content=0;url=smtp/index.html>' > target/doc/index.html &&
-  sudo pip install ghp-import &&
-  ghp-import -n target/doc &&
-  git push -fq https://${TOKEN}@github.com/${TRAVIS_REPO_SLUG}.git gh-pages
-
-env:
-  global:
-    secure: m8BXCWDfBh0Hkajp7+KdSFP65q3gUuJWJSb+W6BtAzWNxIuAwTgtm3Ja3gwwlko+XSOO4IG8+p+ApnduK1znWO+mslhUC7qr8oxIWRS3CzvSr3YeEwfVbWOz34/+I5cGh91ie8pbqxP+buMPVj078W3Cr/STzHhS2ZdQpjR/L8c=