Bump github.com/buildpacks/libcnb from 1.17.0 to 1.18.0

Bumps [github.com/buildpacks/libcnb](https://github.com/buildpacks/libcnb) from 1.17.0 to 1.18.0.
- [Release notes](https://github.com/buildpacks/libcnb/releases)
- [Commits](buildpacks/libcnb@v1.17.0...v1.18.0)

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -5,7 +5,7 @@ go 1.15
 require (
 	github.com/BurntSushi/toml v0.3.1
 	github.com/Masterminds/semver/v3 v3.1.0
-	github.com/buildpacks/libcnb v1.17.0
+	github.com/buildpacks/libcnb v1.18.0
 	github.com/creack/pty v1.1.11
 	github.com/heroku/color v0.0.6
 	github.com/imdario/mergo v0.3.11

go.sum

@@ -4,6 +4,8 @@ github.com/Masterminds/semver/v3 v3.1.0 h1:Y2lUDsFKVRSYGojLJ1yLxSXdMmMYTYls0rCvo
 github.com/Masterminds/semver/v3 v3.1.0/go.mod h1:VPu/7SZ7ePZ3QOrcuXROw5FAcLl4a0cBrbBpGY/8hQs=
 github.com/buildpacks/libcnb v1.17.0 h1:CKQ7Zv8n2OEsdxCVOPyNadolmYme2snwpmznQRAVYqw=
 github.com/buildpacks/libcnb v1.17.0/go.mod h1:VXG/gaLyDE0GS/5Ypa59BDHZTKcFNdVgMrWmTzQhyEs=
+github.com/buildpacks/libcnb v1.18.0 h1:Q7I+HjQ1Cq02/AqhTOrzecuNESip9xE2axckxnymYLQ=
+github.com/buildpacks/libcnb v1.18.0/go.mod h1:yzAQd//jyUXVx6Z/F0cKk/hrl49QZq1OE/hP5+/ZPuQ=
 github.com/creack/pty v1.1.11 h1:07n33Z8lZxZ2qwegKbObQohDhXDQxiMMz1NOUGYlesw=
 github.com/creack/pty v1.1.11/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=