Travis: disable gofmt linter.

goimports is still enabled, and it's a superset of gofmt.

.travis/script

@@ -17,6 +17,7 @@ gometalinter.v2 --install
 # maligned is disabled because of these comments:
 # * https://www.reddit.com/r/golang/comments/3lahav/aligncheck_helps_to_find_inefficiently_packed/cv4u4lg/
 # * https://www.reddit.com/r/golang/comments/3lahav/aligncheck_helps_to_find_inefficiently_packed/cv5wnom/
+# gofmt is disabled because it's a subset of goimports.
 # test is disabled because we already do tests earlier in the script.
 # The --exclude line disables warnings on the portion of x509 that is copied
 # verbatim from the Go standard library.
@@ -28,6 +29,7 @@ gometalinter.v2 --enable-all \
 --disable=errcheck \
 --disable=gas \
 --disable=gocyclo \
+--disable=gofmt \
 --disable=golint \
 --disable=ineffassign \
 --disable=lll \
@@ -49,6 +51,7 @@ echo ""
 echo "gometalinter non-critical (warnings expected):"
 gometalinter.v2 --enable-all \
 --disable=maligned \
+--disable=gofmt \
 --disable=test \
 --concurrency=3 \
 --deadline=10m \