added code to ignore S3 builds when no access key or secret provided

.drone.yml

@@ -1,5 +1,10 @@
 image: go1.2
+env:
+  - GOROOT=/usr/local/go
+  - GOPATH=/var/cache/drone
+  - PATH=$GOPATH/bin:$GOPATH/bin:$PATH
 script:
+  - sudo apt-get -y install libsqlite3-dev sqlite3 mercurial bzr 1> /dev/null 2> /dev/null
   - make deps
   - make
   - make test

pkg/plugin/publish/s3.go

@@ -51,6 +51,14 @@ type S3 struct {
 }
 
 func (s *S3) Write(f *buildfile.Buildfile) {
+
+	// skip if AWS key or SECRET are empty. A good example for this would
+	// be forks building a project. S3 might be configured in the source
+	// repo, but not in the fork
+	if len(s.Key) == 0 || len(s.Secret) == 0 {
+		return
+	}
+
 	// install the AWS cli using PIP
 	f.WriteCmdSilent("[ -f /usr/bin/sudo ] || pip install awscli 1> /dev/null 2> /dev/null")
 	f.WriteCmdSilent("[ -f /usr/bin/sudo ] && sudo pip install awscli 1> /dev/null 2> /dev/null")