Travis: maybe upgrading wheel?

.travis.yml

@@ -1,7 +1,6 @@
 dist: trusty
 language: python
 cache:
-  ccache: true
   pip: true
   directories:
   - $HOME/Library/Caches/Homebrew
@@ -62,7 +61,8 @@ before_cache:
 before_install: |
   mkdir -p bin
   if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
-    pip install --upgrade pip
+    pip3 install --upgrade pip
+    pip3 install --upgrade wheel
     mkdir -p packages
     wget -q 'https://www.dropbox.com/s/vaq0kbwi6e6au80/unpaper_6.1-1.deb?raw=1' -O packages/unpaper_6.1-1.deb
     sudo dpkg -i packages/unpaper_6.1-1.deb