fix environment variables to match README

[natejenkins]

The README asks for the following in ~/.gradle/gradle.properties:

CITYMAPPER_API_KEY=[your API key]
CITYMAPPER_ENDPOINT_URL=[your API endpoint url]
CITYMAPPER_SAMPLE_GOOGLE_MAP_KEY=[your google map key]

However the module build.gradle has:

  buildConfigField "String", "CITYMAPPER_API_KEY", "\"${project.findProperty("SAMPLE_API_KEY") ?: ""}\""
  buildConfigField "String", "CITYMAPPER_ENDPOINT_URL", "\"${project.findProperty("SAMPLE_ENDPOINT_URL") ?: ""}\""

So it is looking for SAMPLE_API_KEY instead of CITYMAPPER_API_KEY.

Modifying these names fixes the demo.