Add dependencies for rgdal

Due to following error on build:

"checking for gdal-config... no
no
configure: error: gdal-config not found or not executable.
"

gdal and prof4 dependencies are added to "before_install" directive

Author: serhatcevikel

.travis.yml

@@ -1,5 +1,7 @@
 before_install:
-  - sudo apt-get install -y libudunits2-dev
+  - sudo add-apt-repository ppa:ubuntugis/ppa -y
+  - sudo apt-get update -q
+  - sudo apt-get install -y libudunits2-dev proj-bin gdal-bin
 
 language: R
 cache: