travis: Add env variable for gox cross-compilation

.travis.yml

@@ -1,4 +1,5 @@
 language: go
+
 go:
   - 1.3.3
   - 1.4.2
@@ -15,15 +16,17 @@ notifications:
     on_success: change
     on_failure: change
 
+env: GOX_OS="linux darwin openbsd freebsd"
+
 install:
   - go get github.com/mitchellh/gox
-  - gox -build-toolchain -os "linux darwin openbsd freebsd"
+  - gox -build-toolchain -os "$GOX_OS"
   - go get -v -t ./...
 
 script:
   - go build -ldflags "-s" ./...
   - go build -ldflags "-s" -o restic ./cmd/restic
-  - sh -c "cd cmd/restic && gox -verbose -os 'linux darwin openbsd freebsd' && ls -al"
+  - sh -c 'cd cmd/restic && gox -verbose -os "$GOX_OS" && ls -al'
   - go test -v ./...
   - ./testsuite.sh
   - sh -c "cd backend && go test -v -test.sftppath /usr/lib/openssh/sftp-server ./..."