chore(travis): use local ghp-import

.travis.yml

@@ -2,8 +2,8 @@ language: rust
 sudo: false
 
 cache:
-  directories:
-    - target
+    directories:
+        - target
 
 script:
     - cargo build --features nightly
@@ -17,7 +17,7 @@ after_success: |
     echo '<meta http-equiv=refresh content=0;url=hyper/index.html>' > target/doc/index.html &&
     git shortlog -s -n | cut -c 8- > target/doc/humans.txt &&
     pip install --user ghp-import &&
-    ghp-import -n target/doc &&
+    /home/travis/.local/bin/ghp-import -n target/doc &&
     git push -fq https://${TOKEN}@github.com/${TRAVIS_REPO_SLUG}.git gh-pages
 
 env: