Try travis buuild matrix feature to install lftp.

This was suggested by the travis support. #513

.travis.yml

@@ -2,22 +2,30 @@ os:
   - linux
   - osx
 
+env:
+  - LFTP_SUPPORT=0
+
+jobs:
+  include:
+    - os: linux
+      env: LFTP_SUPPORT=1
+      addons:
+        apt:
+          packages:
+            - lftp
+    - os: osx
+      env: LFTP_SUPPORT=1
       addons:
         homebrew:
           packages:
             - lftp
 
-env:
-  - LFTP_SUPPORT=0
-  - LFTP_SUPPORT=1
-
 language: sh
 
 install:
   - >
     if [ "$TRAVIS_OS_NAME" = "linux" ]; then
       ./tests/vsftpd-3.0.3.debian7 tests/vsftpd.conf &
-      if [ "$LFTP_SUPPORT" = "1" ]; then sudo apt-get install -y lftp; fi
     fi
   - >
     if [ "$TRAVIS_OS_NAME" = "osx" ]; then