go.{mod,sum}: Bump go-improvmx to 0.15.0 for SMTP support

Author: issyl0

go.mod

@@ -5,7 +5,7 @@ go 1.16
 require (
 	github.com/hashicorp/hcl/v2 v2.8.2 // indirect
 	github.com/hashicorp/terraform-plugin-sdk/v2 v2.7.0
-	github.com/issyl0/go-improvmx v0.14.0
+	github.com/issyl0/go-improvmx v0.15.0
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 	google.golang.org/genproto v0.0.0-20200904004341-0bd0a958aa1d // indirect
 )

go.sum

@@ -185,8 +185,8 @@ github.com/huandu/xstrings v1.3.2/go.mod h1:y5/lhBue+AyNmUVz9RLU9xbLR0o4KIIExikq
 github.com/ianlancetaylor/demangle v0.0.0-20181102032728-5e5cf60278f6/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
 github.com/imdario/mergo v0.3.11/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
 github.com/imdario/mergo v0.3.12/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
-github.com/issyl0/go-improvmx v0.14.0 h1:6gOTBNkqlKJDO7mzj0IazrdvLZzU0uocZlyDA6dtOVY=
-github.com/issyl0/go-improvmx v0.14.0/go.mod h1:vDFMvlPogKkhgcIc4+uVnYnTdpP8w+0ZxjzaI6Lperw=
+github.com/issyl0/go-improvmx v0.15.0 h1:HdY/h/TDi4EZNvCsCj3H3DY6lBcpBdRQ7r9r3XZ8HqI=
+github.com/issyl0/go-improvmx v0.15.0/go.mod h1:vDFMvlPogKkhgcIc4+uVnYnTdpP8w+0ZxjzaI6Lperw=
 github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99/go.mod h1:1lJo3i6rXxKeerYnT8Nvf0QmHCRC1n8sfWVwXF2Frvo=
 github.com/jessevdk/go-flags v1.5.0/go.mod h1:Fw0T6WPc1dYxT4mKEZRfG5kJhaTDP9pj1c2EWnYs/m4=
 github.com/jhump/protoreflect v1.6.0 h1:h5jfMVslIg6l29nsMs0D8Wj17RDVdNYti0vDN/PZZoE=