set Go version to 1.15 and run go mod tidy

go.mod

 module github.com/libp2p/go-libp2p-transport-upgrader
 
-go 1.12
+go 1.15
 
 require (
 	github.com/ipfs/go-log v1.0.4

go.sum

@@ -74,7 +74,6 @@ github.com/libp2p/go-buffer-pool v0.0.2/go.mod h1:MvaB6xw5vOrDl8rYZGLFdKAuk/hRoR
 github.com/libp2p/go-flow-metrics v0.0.3/go.mod h1:HeoSNUrOJVK1jEpDqVEiUOIXqhbnS27omG0uWU5slZs=
 github.com/libp2p/go-libp2p-core v0.3.0/go.mod h1:ACp3DmS3/N64c2jDzcV429ukDpicbL6+TrrxANBjPGw=
 github.com/libp2p/go-libp2p-core v0.5.0/go.mod h1:49XGI+kc38oGVwqSBhDEwytaAxgZasHhFfQKibzTls0=
-github.com/libp2p/go-libp2p-core v0.8.0 h1:5K3mT+64qDTKbV3yTdbMCzJ7O6wbNsavAEb8iqBvBcI=
 github.com/libp2p/go-libp2p-core v0.8.0/go.mod h1:FfewUH/YpvWbEB+ZY9AQRQ4TAD8sJBt/G1rVvhz5XT8=
 github.com/libp2p/go-libp2p-core v0.8.2 h1:/eaSZACWftJZYm07S0nRxdI84v1hSmgnCXrGOvJdpNQ=
 github.com/libp2p/go-libp2p-core v0.8.2/go.mod h1:FfewUH/YpvWbEB+ZY9AQRQ4TAD8sJBt/G1rVvhz5XT8=
@@ -118,7 +117,6 @@ github.com/multiformats/go-multiaddr v0.2.2/go.mod h1:NtfXiOtHvghW9KojvtySjH5y0u
 github.com/multiformats/go-multiaddr v0.3.0/go.mod h1:dF9kph9wfJ+3VLAaeBqo9Of8x4fJxp6ggJGteB8HQTI=
 github.com/multiformats/go-multiaddr v0.3.1 h1:1bxa+W7j9wZKTZREySx1vPMs2TqrYWjVZ7zE6/XLG1I=
 github.com/multiformats/go-multiaddr v0.3.1/go.mod h1:uPbspcUPd5AfaP6ql3ujFY+QWzmBD8uLLL4bXW0XfGc=
-github.com/multiformats/go-multiaddr-net v0.2.0 h1:MSXRGN0mFymt6B1yo/6BPnIRpLPEnKgQNvVfCX5VDJk=
 github.com/multiformats/go-multiaddr-net v0.2.0/go.mod h1:gGdH3UXny6U3cKKYCvpXI5rnK7YaOIEOPVDI9tsJbEA=
 github.com/multiformats/go-multibase v0.0.1/go.mod h1:bja2MqRZ3ggyXtZSEDKpl0uO/gviWFaSteVbWT51qgs=
 github.com/multiformats/go-multibase v0.0.3 h1:l/B6bJDQjvQ5G52jw4QGSYeOTZoAwIO77RblWplfIqk=