.travis.yml: add missing dependency on libboost-filesystem-dev

... to avoid the following build failure on Travis CI:

[ 96%] Linking CXX executable cstrans-df-run
/usr/bin/ld: cannot find -lboost_filesystem
/usr/bin/ld: cannot find -lboost_system
collect2: error: ld returned 1 exit status
make[4]: *** [cstrans-df-run] Error 1

Author: kdudka

.travis.yml

@@ -9,7 +9,7 @@ before_install:
     - sudo apt-get update
 
 install:
-    - sudo apt-get install cmake help2man libboost-dev libboost-program-options-dev libboost-python-dev libboost-regex-dev
+    - sudo apt-get install cmake help2man libboost-dev libboost-filesystem-dev libboost-program-options-dev libboost-python-dev libboost-regex-dev
     - sudo apt-get install -y --force-yes csbuild
 
 script: