Require Go 1.15 or later

The library github.com/golang-jwt/jwt/v4 requires the FillByte() method
of *big.Int, so we're raising the minimum Go version to 1.15.

CONTRIBUTING.md

@@ -66,8 +66,8 @@ Development Environment
 The repository contains the code written for restic in the directories
 `cmd/` and `internal/`.
 
-Restic requires Go version 1.14 or later for compiling. Clone the repo (without
-having `$GOPATH` set) and `cd` into the directory:
+Make sure you have the minimum required Go version installed. Clone the repo
+(without having `$GOPATH` set) and `cd` into the directory:
 
     $ unset GOPATH
     $ git clone https://github.com/restic/restic