fix: Removed import path comments

[hiranya911]

Also bumped the minimum Go version to 1.12.

[lahirumaramba]

Thanks! LGTM!

[hiranya911]

After a bit of experimentation it seems the safest option is to remove the import paths altogether.

1. Import paths are not used when used as a module.

From https://golang.org/cmd/go/#hdr-Remote_import_paths

Import path checking is also disabled when using modules. Import path comments are obsoleted by the go.mod file's module statement.

2. Having import paths set to the versioned (/v4) package names break compilation when used in the non-module mode.

package main

import (
        "context"
	firebase "firebase.google.com/go"
)

func main() {
	app, err := firebase.NewApp(context.Background(), nil)
	// ...
}

main.go:8:2: code in directory gopath/src/firebase.google.com/go expects import "firebase.google.com/go/v4"

The import path is basically making it impossible for non-modules users to import the code.

[lahirumaramba]

Makes sense! Thanks for checking!