minor fix

.travis.yml

@@ -2,7 +2,7 @@ language: csharp
 solution: csharp/SparkCLR.sln
 sudo: required
 dist: trusty
-install:
+before_install:
   - nuget install NUnit.Runners -Version 3.0.0 -OutputDirectory testrunner
   # install maven 3.3.3
   - wget http://archive.apache.org/dist/maven/maven-3/3.3.3/binaries/apache-maven-3.3.3-bin.tar.gz