Refactoring the installation of dep

https://github.com/golang/dep/blob/master/docs/FAQ.md#how-do-i-use-dep-in-ci

Author: Mattia Bertorello

Diff for: .travis.yml

@@ -3,17 +3,26 @@ language: go
 go:
   - 1.11.x
 
+env:
+  - DEP_VERSION="0.5.0"
+
+# Anything in before_script that returns a nonzero exit code will flunk the
+# build and immediately stop. It's sorta like having set -e enabled in bash.
+# Make sure golangci-lint is vendored.
 before_install:
-  - go get -t -v ./...
   - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s -- -b $GOPATH/bin v1.10.2
-  - curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
+  # Download the binary to bin folder in $GOPATH
+  - curl -L -s https://github.com/golang/dep/releases/download/v${DEP_VERSION}/dep-linux-amd64 -o $GOPATH/bin/dep
+  # Make the binary executable
+  - chmod +x $GOPATH/bin/dep
 
-script:
-  - cd $GOPATH/src/github.com/arduino/arduino-cli
+install:
   # Check the dependency, -skip-lock may be useful because the version are not fixed but depends
   # from the branch master of the libraries so they change very often
   - dep check -skip-vendor
   - dep ensure
+
+script:
   # Check if the code is formatted
   - $(exit $(go fmt ./... | wc -l))
   # Run linter
@@ -24,3 +33,7 @@ script:
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)
+
+cache:
+  directories:
+    - $GOPATH/pkg/dep