chore: update rollup with new sourcemap option

Since rollup 0.48.0, the source map options has been renamed
from `sourceMap` to `sourcemap`. This commit add the compatibility
with rollup by detecting one of these two options.

src/index.js

@@ -55,7 +55,12 @@ module.exports = (options = {}) => {
      * @return {void}
      */
     options(opts) {
-      if (opts && opts.sourceMap) {
+      if (!opts) {
+        return;
+      }
+
+      // Rollup >= 0.48 replace `sourceMap` with `sourcemap`.
+      if (opts.sourceMap || opts.sourcemap) {
         plugin.enableSourceMap();
       }
     },

test/index.spec.js

@@ -56,6 +56,18 @@ describe('rollup-plugin-license', () => {
     expect(LicensePlugin.prototype.enableSourceMap).toHaveBeenCalledWith();
   });
 
+  it('should enable sourceMap if options.sourcemap (lowercase) is true', () => {
+    spyOn(LicensePlugin.prototype, 'enableSourceMap').and.callThrough();
+
+    const instance = plugin();
+    const result = instance.options({
+      sourcemap: true,
+    });
+
+    expect(result).not.toBeDefined();
+    expect(LicensePlugin.prototype.enableSourceMap).toHaveBeenCalledWith();
+  });
+
   it('should not enable sourceMap if options.sourceMap is false', () => {
     spyOn(LicensePlugin.prototype, 'enableSourceMap').and.callThrough();