Fixes a Terser bug by locking Terser version to its compatible version

Upgrades to Terser for ES6-compatible minification (#857, @fdintino)

Upgrades Rollup to 0.58 (@laurentgoudet.in cae14f9)

• Fixes regression from SFX interop handling in 0.16.11 (e17be8e)
• Source maps normalization fixes thanks to @thomas-darling (#834)
• Fixes a conditional mapping bug (058aa55, jspm/jspm#2367)

Fixes Object.defineProperty(module.exports assignment in CommonJS module SFX builds (jspm/jspm#2364)

• Update Rollup options to latest specs (jspm/jspm#2341, 2cea073, @Karolusrex)
• Source maps normalization fixes (#823, @thomas-darling)
• Support an uglify.output option to match uglify api (e1b85ca, @CxRes)

__useDefault fixes corresponding to SystemJS 0.20.14

First-stage dynamic import support.

• Fixes handling of control characters in packageConfigPaths (#811)
• Updates SFX build to handle environments without self or global (#812)

Fixes SFX build bug typo (b7b1936)