Bump ims-lti from 1.2.7 to 1.2.8

Bumps [ims-lti](https://github.com/instructure/ims-lti) from 1.2.7 to 1.2.8.
- [Release notes](https://github.com/instructure/ims-lti/releases)
- [Commits](https://github.com/instructure/ims-lti/commits)

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

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

Gemfile.lock

@@ -190,9 +190,10 @@ GEM
     image_processing (1.12.2)
       mini_magick (>= 4.9.5, < 5)
       ruby-vips (>= 2.0.17, < 3)
-    ims-lti (1.2.7)
+    ims-lti (1.2.8)
       builder (>= 1.0, < 4.0)
       oauth (>= 0.4.5, < 0.6)
+      rexml
     influxdb (0.8.1)
     jbuilder (2.11.5)
       actionview (>= 5.0.0)