v1.5-beta3

Updates included in this release:

• OmniSharp has been updated to the recent v1.9-beta18 build, which includes a newer version of the CoreCLR that doesn't crash on Linux kernel >= 4.6.2. (#775, #761, #773)
• It is now possible to dismiss the prompt to generate assets for building and debugging a particularly project permanently. In addition, a new dotnet.generateAssets command has been added to force regeneration of the assets. (#635)
• OmniSharp launch process has been cleaned up a bit to properly stop trying to launch and report an error when it fails. (#804)