Build libwebp instead of using apt package

libwebp-dev package is too old, because travis-ci VMs are based on
Ubuntu 12.04. go-libwebp supports libwebp 4.0 or later.

.travis.yml

@@ -6,4 +6,14 @@ go:
 
 before_install:
   - sudo apt-get update -qq
-  - sudo apt-get install -qq libwebp-dev
+  - sudo apt-get install -qq libjpeg-dev libpng-dev libtiff-dev libgif-dev
+  - > 
+      cd /tmp
+      && wget http://downloads.webmproject.org/releases/webp/libwebp-0.4.2.tar.gz
+      && tar xf libwebp-0.4.2.tar.gz
+      && cd libwebp-0.4.2
+      && ./configure
+      && make
+      && sudo make install
+  - cd $HOME/gopath/src/github.com/harukasan/go-libwebp
+  - export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH