chore: regenerate README (#427)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-04-14 00:47:02,752 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-texttospeech/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-14 00:47:04,052 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/e8305e63-1088-4ef6-9d17-08bca6f9c1c5/targets

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.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-texttospeech.git",
-        "sha": "e97ad7d1ce1bb2749a200492967523c5228c0c4a"
+        "sha": "400ab406955d31994118d0ec31ab30477d2ae034"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
+        "sha": "082e1ca0863b13ada8594fe91845380765da5b70"
       }
     }
   ]

README.md

@@ -39,7 +39,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-texttospeech</artifactId>
-  <version>1.3.0</version>
+  <version>1.4.0</version>
 </dependency>
 
 ```
@@ -52,12 +52,12 @@ compile 'com.google.cloud:google-cloud-texttospeech'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-texttospeech:1.3.0'
+compile 'com.google.cloud:google-cloud-texttospeech:1.4.0'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-texttospeech" % "1.3.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-texttospeech" % "1.4.0"
 ```
 
 ## Authentication