update gradle syntax to support both .gradle and .gradle.kts

[ZZZank]

.gradle.kts, unlike .gradle, is using Kotlin instead of Groovy as the base of its scripting language. The old syntax is problematic in .gradle.kts becasue:

• Kotlin string cannot be wrapped by single quote, aka 'some string' is not allowed and should be "some string" instead
• Kotlin requires brackets for method calling, aka f someArg is not allowed and should be f(someArg) instead

So I made some changes to the syntax so that the installation guide is valid for both .gradle.kts, unlike .gradle

build.gradle.kts:

build.gradle: