Update git requirement from = 1.13.1 to = 1.18.0 (#15)

Updates the requirements on [git](https://github.com/ruby-git/ruby-git) to permit the latest version.
- [Release notes](https://github.com/ruby-git/ruby-git/releases)
- [Changelog](https://github.com/ruby-git/ruby-git/blob/master/CHANGELOG.md)
- [Commits](ruby-git/ruby-git@v1.13.1...v1.18.0)

---
updated-dependencies:
- dependency-name: git
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: .
   specs:
     incr (1.0.0)
-      git (= 1.13.1)
+      git (= 1.18.0)
       gli (= 2.21.0)
       sem_version (= 2.0.1)
 
@@ -13,7 +13,7 @@ GEM
       public_suffix (>= 2.0.2, < 6.0)
     ast (2.4.2)
     diff-lcs (1.5.0)
-    git (1.13.1)
+    git (1.18.0)
       addressable (~> 2.8)
       rchardet (~> 1.8)
     gli (2.21.0)

incr.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
 
   spec.add_runtime_dependency('gli', '2.21.0')
   spec.add_runtime_dependency('sem_version', '2.0.1')
-  spec.add_runtime_dependency('git', '1.13.1')
+  spec.add_runtime_dependency('git', '1.18.0')
 
   spec.add_development_dependency('rake', '13.0.6')
   spec.add_development_dependency('rspec', '3.12.0')