compatibility with 2021.1

[zeitlinger]

Fixes #318

• Wait for feedback from Compatibility with IntelliJ EAP 2021.1 #318
• change "beta" channel to "main" (by removing https://github.com/zalando/intellij-swagger/pull/324/files#diff-49a96e7eea8a94af862798a45174e6ac43eb4f8b4bd40759b5da63ba31ec3ef7R67)
• release to main
• fix travis build (local test worked)

[zeitlinger]

Just installed the beta myself:

java.lang.NullPointerException
	at org.zalando.intellij.swagger.file.icon.SpecIconProvider.getIcon(SpecIconProvider.java:27)
	at com.intellij.util.PsiIconUtil.getProvidersIcon(PsiIconUtil.java:17)
	at com.intellij.ide.FileIconPatcherImpl.getIcon(FileIconPatcherImpl.java:37)
	at com.intellij.util.IconUtil.getProvidersIcon(IconUtil.java:222)
	at com.intellij.util.IconUtil.computeFileIcon(IconUtil.java:155)
	at com.intellij.openapi.fileEditor.impl.EditorsSplitters.lambda$updateFileIconAsynchronously$7(EditorsSplitters.java:419)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.insideReadAction(NonBlockingReadActionImpl.java:521)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.lambda$attemptComputation$3(NonBlockingReadActionImpl.java:468)
	at com.intellij.openapi.application.impl.ApplicationImpl.tryRunReadAction(ApplicationImpl.java:1096)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.lambda$runInReadActionWithWriteActionPriority$0(ProgressIndicatorUtils.java:79)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.runActionAndCancelBeforeWrite(ProgressIndicatorUtils.java:157)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.lambda$runWithWriteActionPriority$1(ProgressIndicatorUtils.java:119)

[jinyule]

org.zalando.intellij.swagger.file.SwaggerUiCreator#copyFromJar
URI resource = getClass().getResource("").toURI(); ==> URI resource = Objects.requireNonNull(getClass().getResource("/" + SWAGGER_UI_FOLDER_NAME)).toURI();