Test against system libwebp on Travis CI

* Dropped custom ppa used only for libwebp backport
* Dropped lossless test from `make check` (not supported in 0.1.3)

.travis.yml

@@ -1,6 +1,5 @@
 language: cpp
 before_install:
-  - sudo add-apt-repository ppa:lyrasis/precise-backports -y
   - sudo apt-get update -qq
   - sudo apt-get install automake gtk-doc-tools 
   - sudo apt-get install gobject-introspection

test/test_formats.sh

@@ -192,7 +192,6 @@ if test_supported jpegload; then
 fi
 if test_supported webpload; then
 	test_format $image webp 90
-	test_format $image webp 0 [lossless]
 fi
 test_format $image ppm 0
 test_format $image pfm 0