ci: replaces travisci with circleci

Travis fails intermittently which blocks us often for merging PRs.

CircleCI was being working great for `graphql-go/graphql` for a while now.

Author: chris-ramon

.circleci/config.yml

@@ -0,0 +1,40 @@
+defaults: &defaults
+  working_directory: /go/src/github.com/graphql-go/handler
+  steps:
+    - checkout
+    - run: go get -v -t -d ./...
+    - run: go test ./...
+ 
+version: 2
+jobs:
+  golang:1.8.7:
+    <<: *defaults
+    docker:
+      - image: circleci/golang:1.8.7
+  golang:1.9.7:
+    <<: *defaults
+    docker:
+      - image: circleci/golang:1.9.7
+  golang:latest:
+    <<: *defaults
+    docker:
+      - image: circleci/golang:latest
+  coveralls:
+    working_directory: /go/src/github.com/graphql-go/handler
+    docker:
+      - image: circleci/golang:latest
+    steps:
+      - checkout
+      - run: go get -v -t -d ./...
+      - run: go get github.com/mattn/goveralls
+      - run: go test -v -cover -race -coverprofile=coverage.out
+      - run: /go/bin/goveralls -coverprofile=coverage.out -service=circle-ci -repotoken $COVERALLS_TOKEN
+
+workflows:
+  version: 2
+  build:
+    jobs:
+      - golang:1.8.7
+      - golang:1.9.7
+      - golang:latest
+      - coveralls