Bump OpenEXR version to get <cstdint> fix #236

Job	Run time
Build and test g++-9 -	5m 10s
Build and test g++-10 -	4m 24s
Build and test clang++-10 -	4m 43s
Build and test clang++-11 -	4m 55s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	3m 35s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	3m 40s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	4m 3s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	3m 23s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	4m 10s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 34s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 46s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 44s
	49m 7s

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