Missing dependencies for gdal and proj.4 added

Missing dependencies for gdal and proj.4 added as per https://community.rstudio.com/t/can-not-install-rgdal/33971/3

Author: serhatcevikel

.travis.yml

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