circleci: replace Linux tests on Travis CI with CircleCI

CircleCI is faster and has more features than Travis CI. Additionally,
based on the recent news, the future of Travis CI is rather uncertain.

Keep using Travis CI for macOS testing at the moment, as open source
projects will need to get special permission to use CircleCI for macOS
tests.

.travis.yml

@@ -2,31 +2,13 @@ language: go
 
 matrix:
   include:
-  - dist: xenial
-    go: "1.11"
   - os: osx
     go: "1.11"
     env: PATH="/usr/local/opt/llvm/bin:$PATH"
+    before_install:
+    - mkdir -p /Users/travis/gopath/bin
 
 addons:
-  apt:
-    sources:
-    - sourceline: 'ppa:ubuntu-toolchain-r'
-    - sourceline: 'deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-7 main'
-      key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
-    packages:
-    - llvm-7-dev
-    - clang-7
-    - libclang-7-dev
-    - gcc-arm-linux-gnueabihf
-    - binutils-arm-none-eabi
-    - libc6-dev-armhf-cross
-    - gcc-aarch64-linux-gnu
-    - libc6-dev-arm64-cross
-    - qemu-system-arm
-    - qemu-user
-    - gcc-avr
-    - avr-libc
   homebrew:
     update: true
     taps: ArmMbed/homebrew-formulae
@@ -36,7 +18,6 @@ addons:
     - arm-none-eabi-gcc
 
 install:
-  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then mkdir -p /Users/travis/gopath/bin; fi
   - curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
   - dep ensure --vendor-only
 
@@ -52,8 +33,6 @@ script:
   - tinygo build -size short -o test.nrf.elf       -target=nrf52840-mdk examples/blinky1
   - tinygo build -size short -o blinky1.nrf51d.elf -target=pca10031     examples/blinky1
   - tinygo build -size short -o blinky1.stm32.elf  -target=bluepill     examples/blinky1
-  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then tinygo build -size short -o blinky1.avr.elf    -target=arduino      examples/blinky1; fi
-  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then tinygo build -size short -o blinky1.avr.elf    -target=digispark    examples/blinky1; fi
   - tinygo build -size short -o blinky1.reel.elf   -target=reelboard    examples/blinky1
   - tinygo build -size short -o blinky2.reel.elf   -target=reelboard    examples/blinky2
   - tinygo build -size short -o blinky1.pca10056.elf    -target=pca10056     examples/blinky1