Bump tokio-executor from 0.1.6 to 0.1.8

[dependabot-preview]

Bumps tokio-executor from 0.1.6 to 0.1.8.

Changelog

Sourced from tokio-executor's changelog.

0.1.8 (August 23, 2018)

• Extract tokio::executor::current_thread to a sub crate (#370)
• Add Runtime::block_on (#398)
• Add runtime::current_thread::block_on_all (#477)
• Misc documentation improvements (#450)
• Implement std::error::Error for error types (#501)

0.1.7 (June 6, 2018)

• Add Runtime::block_on for concurrent runtime (#391).
• Provide handle to current_thread::Runtime that allows spawning tasks from
  other threads (#340).
• Provide clock::now(), a configurable source of time (#381).

Commits

• 0720340 Bump version to v0.1.8 (#566)
• 8bf2e9a Introduce Timeout and deprecate Deadline. (#558)
• cf184eb timer: Reduce size of Delay struct (#554)
• d822b72 Add DelayQueue implementation to tokio-timer (#550)
• c66b56c Implement Default for tokio_timer::Handle (#553)
• 89639ec Bump tokio-uds version to v0.2.1 (#552)
• 2b1b0ac Expose tokio_uds from the root crate (#526)
• 6b84c73 tokio::codec docs + additional exports (#546)
• 767b370 Add tokio-tls echo example. (#541)
• 28010b5 Update lines_encoder test to use LinesCodec (#544)
• Additional commits viewable in compare view

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