Update minimal go version and tests

Go 1.13 introduced a new interface in the standard library to wrap
errors and the %w modifier to fmt.Errorf which we couldn't use before
since our go.mod declares go 1.12

Signed-off-by: Anatole Denis <anatole@unverle.fr>

.travis.yml

@@ -8,7 +8,6 @@ dist: bionic
 go:
   - 1.14.x
   - 1.13.x
-  - 1.12.x
   - master
 
 env:

go.mod

@@ -1,6 +1,6 @@
 module github.com/coredhcp/coredhcp
 
-go 1.12
+go 1.13
 
 require (
 	github.com/chappjc/logrus-prefix v0.0.0-20180227015900-3a1d64819adb