ci: bump travis, appveyor and module versions

Also remove redundant Go version build tag.

.travis.yml

@@ -5,8 +5,8 @@ language: go
 # Do not move these lines; they are referred to by README.md.
 # Versions of go that are explicitly supported by Gonum, plus Go tip.
 go:
+ - 1.15.x
  - 1.14.x
- - 1.13.x
  - master
 
 os:

appveyor.yml

@@ -1,27 +1,24 @@
 build: off
 
+image: Visual Studio 2019
+
+stack: go 1.14
+
 clone_folder: c:\gopath\src\gonum.org\v1\gonum
 
 cache:
   - '%LocalAppData%\go-build'
 
+environment:
+  GO111MODULE: 'on'
+
 branches:
   only:
     - master
 
-environment:
-  # Do not move these lines; they are referred to by README.md.
-  # Versions of go that are explicitly supported by gonum.
-  matrix:
-    - GOROOT: 'c:\go113'
-    #- GOROOT: 'c:\go114'
-  GOPATH: c:\gopath
-  GOTOOLDIR: '%GOROOT%\pkg\tool\windows_amd64'
-  PATH: '%GOPATH%\bin;%GOROOT%\bin;%PATH%'
-
 build_script:
-  - "%GOROOT%\\bin\\go version"
-  - "%GOROOT%\\bin\\go get -v -t ./..."
+  - go version
+  - go get -v -t ./...
 
 test_script:
-  - "%GOROOT%\\bin\\go test ./..."
+  - go test ./...

go.mod

@@ -1,6 +1,6 @@
 module gonum.org/v1/gonum
 
-go 1.13
+go 1.14
 
 require (
 	golang.org/x/exp v0.0.0-20190125153040-c74c464bbbf2

version.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.12
-
 package gonum
 
 import (