Use Go 1.15 (slackhq#277)

Update all CI checks and release process to use the latest patch version
of go1.15.

.github/workflows/gofmt.yml

@@ -14,10 +14,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.14
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/release.yml

@@ -10,10 +10,10 @@ jobs:
     name: Build Linux All
     runs-on: ubuntu-latest
     steps:
-      - name: Set up Go 1.14
+      - name: Set up Go 1.15
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
 
       - name: Checkout code
         uses: actions/checkout@v2
@@ -34,10 +34,10 @@ jobs:
     name: Build Windows amd64
     runs-on: windows-latest
     steps:
-      - name: Set up Go 1.14
+      - name: Set up Go 1.15
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
 
       - name: Checkout code
         uses: actions/checkout@v2
@@ -58,10 +58,10 @@ jobs:
     name: Build Darwin amd64
     runs-on: macOS-latest
     steps:
-      - name: Set up Go 1.14
+      - name: Set up Go 1.15
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
 
       - name: Checkout code
         uses: actions/checkout@v2

.github/workflows/smoke.yml

@@ -18,10 +18,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.14
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory

.github/workflows/test.yml

@@ -18,10 +18,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.14
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory
@@ -48,10 +48,10 @@ jobs:
         os: [windows-latest, macOS-latest]
     steps:
 
-    - name: Set up Go 1.14
+    - name: Set up Go 1.15
       uses: actions/setup-go@v1
       with:
-        go-version: 1.14
+        go-version: 1.15
       id: go
 
     - name: Check out code into the Go module directory