migrate from travis ci to github actions (resolves #8)

Author: tayloraswift

.github/workflows/build.yml

@@ -0,0 +1,55 @@
+
+name: build
+
+on:
+    push:
+        branches: [ master ]
+    pull_request:
+        branches: [ master ]
+
+jobs:
+    build-macos:
+        runs-on: ${{ matrix.os }}
+        strategy:
+            matrix:
+                os: [macos-12, macos-11]
+        steps:
+            -   uses: actions/checkout@v2
+            -   name: build 
+                run: |
+                    swift --version
+                    swift build
+    build-linux:
+        runs-on: ${{ matrix.os }}
+        strategy:
+            matrix:
+                os: [ubuntu-20.04, ubuntu-18.04]
+                swift: [5.6.2, 5.5.3]
+        steps:
+            -   uses: actions/checkout@v2
+            
+            -   name: cache swift toolchains
+                uses: actions/cache@v2
+                with:
+                    path: swift-${{ matrix.swift }}.tar.gz
+                    key: ${{ matrix.os }}:swift:${{ matrix.swift }}
+                    
+            -   name: cache status
+                id:   cache_status
+                uses: andstor/file-existence-action@v1
+                with:
+                    files: swift-${{ matrix.swift }}.tar.gz
+            
+            -   name: download swift toolchain 
+                if: steps.cache_status.outputs.files_exists == 'false'
+                run: curl https://download.swift.org/swift-${{ matrix.swift }}-release/$(echo ${{ matrix.os }} | sed 's/[^a-zA-Z0-9]//g')/swift-${{ matrix.swift }}-RELEASE/swift-${{ matrix.swift }}-RELEASE-$(echo ${{ matrix.os }} | sed 's/[^a-zA-Z0-9\.]//g').tar.gz --output swift-${{ matrix.swift }}.tar.gz
+            
+            -   name: set up swift
+                run: |
+                    mkdir -p $GITHUB_WORKSPACE/swift-${{ matrix.swift }}
+                    tar -xzf swift-${{ matrix.swift }}.tar.gz -C $GITHUB_WORKSPACE/swift-${{ matrix.swift }} --strip 1
+                    echo "$GITHUB_WORKSPACE/swift-${{ matrix.swift }}/usr/bin" >> $GITHUB_PATH
+            -   name: build 
+                run: |
+                    swift --version
+                    swift build

.travis.yml

@@ -1,4 +1,4 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.5
 
 import PackageDescription