Bumps version to 1.11.8-SNAPSHOT.

[tgregg]

NOTE: the v1.11.7 release did not include all commits present on the master branch at the time of release. The following commits were excluded from 1.11.7 and will be included in a future release:

• Implements patch list as a single contiguous array of reusable PatchPoint instances. #521
• Remove the repeated logic of the method that overwrites the lower nibble of container type descriptor. #548
• Adds support for writing Ion 1.1 null, bool, int, and float binary encoding #612
• Adds support for writing binary Ion 1.1 timestamps #618
• Adds support for writing Ion 1.1 decimal binary encoding #636
• Adds support for text and lob types and some annotations #637
• Enable configurable write buffer block size. #650
• Enable auto-flush in ion binary writer. #651

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.