Update .travis.yml to use explicit blackfriday version

.travis.yml

@@ -3,8 +3,19 @@ language: go
 sudo: false
 
 install:
-  - mkdir -p "${GOPATH%%:*}/src/github.com/shurcooL/markdownfmt"
-  - git clone https://github.com/tianon/markdownfmt.git "${GOPATH%%:*}/src/github.com/shurcooL/markdownfmt"
+  - export GOPATH_FIRST="${GOPATH%%:*}"
+  - >
+      git clone
+        --depth 1
+        https://github.com/tianon/markdownfmt.git
+        "$GOPATH_FIRST/src/github.com/shurcooL/markdownfmt"
+  - export BLACKFRIDAY_VERSION="$(awk '$1 == "ENV" && $2 == "BLACKFRIDAY_VERSION" { print $3; exit }' "$GOPATH_FIRST/src/github.com/shurcooL/markdownfmt/Dockerfile")"
+  - >
+      git clone
+        --depth 1
+        -b "$BLACKFRIDAY_VERSION"
+        https://github.com/russross/blackfriday.git
+        "$GOPATH_FIRST/src/github.com/russross/blackfriday"
   - go get -v github.com/shurcooL/markdownfmt
 
 script: