Bump combine_pdf from 1.0.26 to 1.0.28

Bumps [combine_pdf](https://github.com/boazsegev/combine_pdf) from 1.0.26 to 1.0.28.
- [Release notes](https://github.com/boazsegev/combine_pdf/releases)
- [Changelog](https://github.com/boazsegev/combine_pdf/blob/master/CHANGELOG.md)
- [Commits](boazsegev/combine_pdf@v1.0.26...v1.0.28)

---
updated-dependencies:
- dependency-name: combine_pdf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -156,7 +156,7 @@ GEM
     choice (0.2.0)
     climate_control (1.2.0)
     coderay (1.1.3)
-    combine_pdf (1.0.26)
+    combine_pdf (1.0.28)
       matrix
       ruby-rc4 (>= 0.1.5)
     concurrent-ruby (1.3.4)