traivs: set depth to false to clone whole repo

Overwrite travis's default of only cloning the last 50 commands so
that travis can find the reference point for our linter.

As in lightningnetwork/lnd/pull/4026

.travis.yml

@@ -7,6 +7,11 @@ cache:
     - $GOPATH/src/github.com/golang
     - $GOPATH/src/gopkg.in/alecthomas
 
+# Remove Travis' default flag --depth=50 from the git clone command to make sure
+# we have the whole git history, including the commit we lint against.
+git:
+  depth: false
+
 go:
   - "1.13.x"